Extensive Model Testing of a Dry-Tree Spread-Moored Barge in Brazillian Waters

This paper describes the overall behavior of a new build barge to be spread moored offshore Brazil in 1,200 m water depth. The barge supports 12 dry trees located in a well bay near amidships. Experiments have been carried out on a scaled model of the barge in survival environments ranging from 4.7m significant beam waves to 7.8m significant bow quartering waves in the presence of wind and strong current. This model test campaign has been partly funded by five oil companies.
Extensive instrumentation was used to provide information on vessel and risers during these tests. The barge motion behavior was monitored to allow assessment of the floater sea-keeping properties and of the riser/hull clearance. Wave gauges fixed to the barge measured the moonpool agitation and its effects, if any, on the risers. Riser top tensions were monitored to determine their dynamics and to verify the over-tension factor, which can be compared to other dry tree solutions. Finally, various loads in the tensioning system were monitored and provide valuable information for the hull structural design.
The extensive model test campaign has confirmed 1) the feasibility of the Tension Leg Deck (TLD) system to pretension risers offshore Brazil; 2) the seakeeping performance of the spread moored barge supporting these risers; 3) the riser/tendon clearance and 4) the wave agitation in the moonpool.